The Deepwater Horizon oil rig exploded in April 2010, killing 11 crew members and pouring oil into the Gulf of Mexico for 87 days.

This is the clearest sign yet that the U.S. government is seeking a larger settlement than previously expected over claims related to the spill. A ruling of gross negligence would raise BP's financial liability from $4.5 billion to $21 billion under the Clean Water Act, according to Reuters.

All the while, the oil firm's lawyers have rejected the charge of gross negligence. "BP believes it was not grossly negligent and looks forward to presenting evidence on this issue at trial in January," the firm said in a statement, according to Reuters.

The case is planned to go to trial in New Orleans in January 2013.

BP shares fell 3.7 percent in London trading on Wednesday, according to The Financial Times.